This invention is directed to methods of and apparatus for monitoring gaseous atmospheres and, more particularly, to methods of and apparatus for monitoring the concentration of combustible gases in an atmosphere.
In recent years, major progress has been made toward the development of pulping apparatus for delignifying wood by the application of gaseous oxygen in an aqueous media. Of the proposed oxygen pulping systems, two-stage soda-oxygen pulping has received the most attention. In two-stage soda-oxygen pulping a first stage soda or alkaline delignification is followed by a second stage alkaline oxygen delignification. Preferably, the second stage delignification is performed in a recirculating atmosphere oxygen reactor. While a recirculating atmosphere oxygen reactor is preferred for cost reasons, the recirculation of the reactor atmosphere creates a potentially hazardous situation due to the gaseous and vaporous combustible products formed during the delignification process. More specifically, since the reactor atmosphere is recirculated, the concentration of combustible pulp degradation products, as well as volatile pulp and dilution zone constituents, in the gas phase, increases with time. The increase is such that after a period of time a combustible (explosive) level may be reached, unless the reactor atmosphere is vented, or the combustibles are removed in some manner. This time frame, which relates to the rate at which the foregoing items are produced, has been found to extend from a few hours to a few days, depending upon reactor design and conditions.
As those skilled in the art and others will readily recognize, there are two well-defined limits within which self-propagation of a flame (e.g. a deflagrative explosion) will take place upon ignition. These limits are defined as the upper explosive limit (UEL) and the lower explosive limit (LEL). The LEL is defined as the lowest concentration (% vol. basis) of a combustible gas that, when ignited with an open flame or spark, will propagate a flame. As the concentration of gases increases, a point is reached whereat the gas will burn at the point of ignition, but will not propagate a flame. This concentration is defined as the UEL. (These limits are sometimes also referred to as the upper and lower limits of inflammability).
It will be appreciated that, in order to prevent a potentially inflammable atmosphere from reaching its LEL, the concentration of combustible materials in the gas must be kept well below the LEL by, for example, effective venting to the earth's atmosphere. The present invention is directed to providing a method of and apparatus for monitoring a potentially inflammable atmosphere, particularly the atmosphere of an oxygen reactor used in an oxygen pulping system, to provide information about the concentration of combustibles in the reactor atmosphere. The information may take the form of a record, alarms, or a combination thereof, and may be used in combination with a control system to automatically control apparatus, such as a vent valve, adapted to reduce the concentration of combustible gases in the potentially inflammable atmosphere.
Therefore, it is a general object of this invention to provide a new and improved apparatus for monitoring the presence of combustible gases in a gaseous atmosphere.
It is a further object of this invention to provide a method of and apparatus for monitoring the presence of combustible gases in the atmosphere of an oxygen pulping reactor so as to provide information about the concentration of combustible gases in the reactor atmosphere such that action can be taken to reduce the concentration when it reaches a predetermined level.
In the past, catalytic combustibles analyzers have frequently been used to measure the concentration of combustibles in a gaseous atmosphere. While such devices are useful in certain environments, they possess serious disadvantages, which make them unsuitable for use in other environments. One particular disadvantage becomes significant when the atmosphere includes a variety of combustible components, such as exists in the atmosphere of an oxygen pulping reactor. Specifically, while catalytic combustibles analyzers are relatively accurate and precise when a single combustible component is contained in the atmosphere being analyzed, they become relatively imprecise when the atmosphere includes a variety of combustible components, because they respond to different components with different sensitivies and do not provide an output related to the concentration or percent LEL of the overall atmosphere. This result is particularly true when the atmosphere includes a variety of organic combustible vapors and carbon monoxide. A further disadvantage of catalytic combustibles analyzers is the rapid poisoning of the catalyst that occurs in the presence of a sulphur-containing gas.
Therefore, it is a further object of this invention to provide a more precise method of and apparatus for monitoring an atmosphere to provide information related to the concentration of combustible gases in the atmosphere.
It is a still further object of this invention to provide a new and improved method of and apparatus for determining when the concentration of combustible gases in an atmosphere has reached a predetermined percent of the lower explosive limit of the atmosphere.
It is a still further object of this invention to provide a new and improved method of and apparatus for monitoring the atmosphere of an oxygen pulping reactor to provide an indication related to the concentration of combustible gases in the atmosphere in terms of percent LEL.